This invention relates to sealing compositions and, in particular, to compositions for sealing alumina ceramic packages in which integrated circuits or other solid state devices are encapsulated.
A known sealing composition for sealing alumina ceramic packages comprises a mixture of vitreous solder glass powder, lead titanate powder and zircon (zirconium silicate ZrSiO.sub.4) powder, as disclosed in a Japanese patent publication No. 56-49861. This known sealing composition has good mechanical strength and resistance to thermal shock, but radiates .alpha.-rays because the zircon includes uranium and/or thorium therein as impurities whereby certain integrated circuits are damaged.
Other sealing compositions are known from Japanese patent publication No. 50-69242 and U.S. Pat. No. 4,310,357. The former discloses a mixture of vitreous glass powder and cordierite powder including .beta.-eucryptite, .beta.-spodumene, zircon and/or lead titanate as optional ingredients, and the latter discloses a mixture of vitreous glass powder and willemite. These sealing compositions are not so high in mechanical strength, and have a tendency to suffer from micro-cracks caused by heat shock resulting in damage to the hermetical seal.